1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
A multicolor image forming apparatus such as a copier, a facsimile, a printer, a plotter, a multifunctional peripheral (MFP), or the like is known, in which toner images of at least two colors are superimposed on a single photosensitive element by using developers of at least two colors through repeating charging and exposing the photosensitive element to form a latent image thereon and developing the latent image, thereby forming a color image. Such multicolor image forming apparatus is disclosed, for example, in Japanese Patent No. 3250851 and Japanese Published Examined Patent Application No. H08-3673.
Recently, a demand for a full-color image forming apparatus has been increasing along with remarkable improvement in quality and cost reduction of color images as above including full-color images that are realized by forming toner images on a single photosensitive element in a superimposing manner (superimposing development). When a color image is formed by using a single photosensitive element in the above manner, the number of times of transferring can be reduced, which results in reducing possibility of degradation of image quality. Moreover, the number of photosensitive elements can be reduced, so that the size of the apparatus can be reduced by the space for the reduced photosensitive elements, thereby reducing the cost for manufacturing the apparatus.
For the conventional superimposing development process, various methods have been proposed to reduce and compensate influence of a toner layer already developed on a photosensitive element.
In the technology disclosed in Japanese Patent No. 3250851, when a plurality of toner images of different colors is formed on a photosensitive element, a charge potential for forming a first color toner image is made different from that for a second color toner image, and a surface potential of the photosensitive element in a portion on which a toner image is formed is made lower than that in a portion on which the toner image is not formed, i.e., a charge potential forming condition is changed, thereby preventing undesired color mixture of toner.
In the technology disclosed in Japanese Published Examined Patent Application No. H08-3673, the thickness of a photosensitive element, a toner layer already formed on the photosensitive element, and a toner layer on a developer carrier (toner carrier), and their relationship are defined, and a developing condition is set to suppress discharging in consideration of color turbidity of an image and color mixture of toner in a developing unit due to reverse transfer.
However, in the technology of Japanese Patent No. 3250851, the toner layer potential increases by superimposing toner images of four colors, so that a developing potential between a toner-layer formed area and a toner-layer non-formed area becomes different, thereby changing image density and hue.
Moreover, in the technology of Japanese Published Examined Patent Application No. H08-3673, when toner images of four colors are superimposed on the photosensitive element, it is expected that the change of a developing potential by the potential of a previously formed toner layer when the thickness of the toner layer increases causes image density and hue to change.
In the image forming apparatus in which toner images of different colors are formed on a single image carrier (photosensitive element) in a superimposing manner as described above, when a toner layer on the image carrier is too thick, the toner images are difficult to superimpose in the developing process due to toner layer potential and light shielding effect. Therefore, the toner layer potential needs to be reduced, and the optical transparency needs to be ensured.
To cope with the above problems, the inventors have reached a conception of an image forming apparatus in which multicolor toner image forming process by using a multicolor toner image forming unit and a transferring/fixing process by using a transferring/fixing unit or the like for simultaneously performing transferring and fixing of a color image are organically combined.
As a result of vigorous investigation, the inventors have achieved a toner-layer filling-rate lowering unit that keeps the toner-layer filling rate of toner on the photosensitive element to relatively low to develop a color toner image while securing the optical transparency in exposure and a transferring/fixing unit that causes a uniform image to be formed in the state where the toner-layer filling rate is relatively low in the transferring and fixing process, i.e., a toner rolling rate adjusting unit that rolls superimposed multicolor toner images with a predetermined toner rolling rate. Therefore, appropriate developing potential can be ensured in the image forming process for the second and successive colors.
In the present invention, a multicolor toner image is formed on the photosensitive element in a state where the toner-layer filling rate is set relatively low that is insufficient for a typical image to obtain sufficient optical transparency in exposure, and the formed multicolor toner image is rolled at a predetermined toner rolling rate by utilizing the toner rolling effect in the transferring and fixing process to compensate the insufficient toner adhering amount. This is different from the conventional technology in which toner with relatively high density is used for forming a multicolor toner image.